1. Field of the Invention
The present invention relates to an image projecting device for optically projecting the image of a document onto a screen and, more particularly, to a plain paper projector (PPP) capable of projecting the image of a document implemented by a plain paper.
2. Discussion of the Background
It is a common practice with a conventional OHP (Over Head Projector) or a slide projector to use a transparent sheet, i.e., an OHP sheet or a slide sheet, as taught in Japanese Patent Laid-Open Publication No. 4-128734 by way of example. When light transmitted through the transparent sheet is projected onto a screen, an image provided on the sheet is projected in an enlarged scale. However, this kind of projecting device is not operable with a document implemented by an opaque plain paper, not to speak of a thick book document. With such an extra document, the user must write an image on the transparent sheet by hand or transfer it to the sheet by use of a copier, consuming time and labor.
In light of the above, Japanese Patent Laid-Open Publication No. 4-149417, for example, proposes a semioptical and semielectrical projecting device capable of projecting light transmitted through an electrically driven TFT (Thin Film Transistor) type liquid crystal display device. This type of projecting device photoelectrically transduces the image of a document, electrically writes the transduced image in the display device, and then optically projects the image on a screen in an enlarged scale. The device has another advantage that the operation for replacing the document is not necessary because images to be projected are transformed to electrical signals beforehand.
However, a problem with the projecting device using the TFT type display device is that because the display device is driven electrically, a scanner for reading a document and a digital image processing device are bulky and expensive. Another problem is that the device is troublesome to operate. In addition, because the display device requires a TFT for every pixel, it is difficult provided it with a great size due to the limited yield.
There has been proposed a PPP capable of transferring the image of a plain paper document to a composite polymer and liquid crystal film, which is a specific form of a liquid crystal film responsive to light, via a lens array, and then projecting the image transferred to the film. However, the focal depth available with a lens array is too small to avoid blurring when the document slightly rises above a platen or has a tridimensional configuration.